In recent years, a fiber laser light source has received much attention as a near infrared laser light source, instead of a conventional solid state laser light source. This is because the fiber laser light source has a high oscillation efficiency and an excellent beam quality, and further, can be cooled with air and has a simple structure.
FIG. 13 is a schematic diagram showing a configuration of a typical fiber laser light source. Laser light emitted from an exciting (pumping) LD (laser diode) 101 is incident upon a rare-earth-doped clad pumping fiber 103 as a laser medium. Then, it is resonated in a laser resonator made up of fiber gratings 102 and 104 as a reflecting mirror and thereby oscillated.
A polarizer 105 is inserted for unifying the polarization directions of the oscillated laser light.
This fiber laser light source has an excellent beam quality and can regulate an oscillation wavelength spectrum with the line width of a reflection spectrum in the fiber grating 104 on the exit side. Hence, the fiber laser light source as a fundamental wave light source is extremely suitable for harmonic generation (called a wavelength conversion light source) provided with a non-linear optical crystal.
A second-harmonic generation (SHG) module 108 shown in FIG. 13 is a mechanism for conducting second harmonic generation. This mechanism contributes to emitting a twofold second harmonic 107 at last.
Furthermore, a laser crystal provided in a conventional solid state laser regulates a laser oscillation wavelength. In contrast, the set of fiber gratings 102 and 104 regulate an oscillation wavelength as well in this fiber laser. In other words, they can vary an oscillation wavelength arbitrarily, though its gain differs according to the wavelength.
On the other hand, a laser display has been widely known as the application of such laser light harmonic to a light source (wavelength conversion light source) (Non-patent Document 1).
Comparing to conventional use of a white lamp, unnecessary infrared rays or ultraviolet rays are less generated, thus keeping the power consumption low. Further, a more efficient light concentration is feasible using laser light, thereby enhancing the light utilization efficiency.
Moreover, compared to the case of a light emitting diode, laser light is monochromatic light and higher in color purity. This helps improve the color reproducibility of a display apparatus. Particularly, deeper green can be displayed by setting the wavelength of green light to 520 to 535 nm.
FIG. 14 shows a color reproduction range of used green light according to a wavelength in the case of blue light having a wavelength of 460 nm and red light having a wavelength of 635 nm on a chromaticity diagram. With respect to wavelengths, a solid state laser can generate only two wavelengths, i.e., a wavelength of 532 nm in the case of using Nd:YAG, Nd:YVO4 or the like, and a wavelength of 527 nm in the case of using Nd:YLF. Particularly, YLF is a fluoride crystal and hard to produce. This makes promising a fiber laser having a broad fluorescence spectrum and capable of selecting an oscillation wavelength freely (Non-patent Document 2).
In a fiber laser or a fiber amplifier, excitation light and oscillation light propagate through the same fiber. As mentioned with reference to Patent Document 1, a part of oscillation light turns into undesirable return light, which may damage an excitation light source. For this reason, use of a lens system and a mirror as shown in FIG. 15 has been studied to eliminate such oscillation light.
It is preferable that the wavelength of a green light source for a laser display apparatus is 530 to 520 nm from the viewpoint of a color reproduction range. However, in the use of a wavelength conversion light source provided with a fiber laser as a fundamental wave light source, light having a wavelength of 1075 nm or less as a fundamental wave within the above-mentioned wavelength range is absorbed into a rare-earth-added fiber as a laser medium. This makes the oscillation operation of a laser resonator unstable. Consequently, the length of a fiber as an interaction length cannot be made greater. This phenomenon is conspicuous in a polarization maintaining fiber such as a PANDA (polarization-maintaining and absorption-reducing) fiber used for attaining a linear polarization necessary to a wavelength conversion light source.
Also, excitation light must be increased to raise the output of laser light. However, depending upon the wavelength of excitation light, excitation light which is absorbed in a rare-earth-added fiber as a laser medium may deteriorate the fiber. FIG. 16 shows a mechanism of such deterioration.
FIG. 16 shows a fusion joint 410 of a double-clad polarization maintaining fiber added with a rare earth and a usual single-mode polarization maintaining fiber. In the double-clad polarization maintaining fiber, residual excitation light 408 is confined in an outer clad 402. In this state, light propagates in an inner clad 403.
On the other hand, in the portion of the connected single-mode polarization maintaining fiber where a coating 407 is removed, air works as a clad, consequently confining residual excitation light 408. However, pumping light leaks from the portion having the coating 407 and its energy generates heat in a part of the single-mode polarization maintaining fiber, e.g., a heat generated part 409. This causes deterioration of the fiber.
For example, in the case where excitation light has 10 W, the absorption of the double-clad polarization maintaining fiber added with a rare earth Yb is 0.6 dB/m. Hence, it will be seen that a fiber having a length of 10 m absorbs excitation light of 7.5 W. Consequently, light having 2.5 W and 915 nm is radiated as residual excitation light and propagated through the clad of the single-mode polarization maintaining fiber.
In the case of the conventional configuration shown in FIG. 13, pumping is conducted with excitation light (915 nm) of 15 W and the output of oscillation light (1064 nm) is 6.8 W. In this case, twenty minutes after a continuous operation, a fusion joint 110 and a primary coat (coating) of a single-mode polarization maintaining fiber 112 were overheated, thus causing deterioration of the fiber.
FIG. 17 is a plotting graph showing a relationship between the fiber length of a Yb-doped double-clad fiber as a laser medium and residual excitation light, using the power of excitation light as a parameter. Some conventional examinations prove that the fiber is deteriorated when the residual excitation light exceeds 3.5 to 4 W. As can be seen from FIG. 17, the excitation light power needs to be made smaller if the fiber length has to be shortened because of a loss possessed by the fiber. In other words, at a wavelength of 1050 nm or 1030 nm at which the fiber has a great loss, an output to be generated is inevitably limited.
In the case of generating oscillation light having a wavelength of 1070 nm or more, which is not absorbed by a fiber, the rare-earth-added double-clad polarization maintaining fiber can be prevented from being overheated by increasing the length thereof. However, at a wavelength of 1060 nm, 1050 nm or the like where green color can be produced by a wavelength conversion, the problems have been known that the loss which is caused by absorption of a fiber increases as the length of a rare-earth-added double-clad polarization maintaining fiber increases, the oscillation becomes unstable, the oscillation cannot be performed at a desired wavelength. Therefore, the intensity of excitation light is obviously determined to keep a fiber from being overheated, thus limiting the maximum output.
FIG. 18 shows an absorption spectrum of a rare-earth-added double-clad fiber which is added with Yb as a rare earth at approximately 1000 ppm as an example of solving these problems.
A laser diode (LD) around 915 nm or a laser diode around 976 nm can be used as excitation light. The fiber absorbs the light of 915 nm at some 0.6 dB/m while it absorbs the light of 976 nm at some 1.8 dB/m. The latter is about three times as great as the former. Thus, it will be seen that the use of the light of 976 nm solves the deterioration of the fiber.
However, the profile of the absorption peak at 976 nm is steep while the profile of the absorption peak at 915 nm is broad. Hence, the 915-nm band (900 to 950 nm) is more stable against a variation in the wavelength of excitation light which is likely to be caused by a change in the temperature of an excitation light LD or the like. This helps simplify a cooling mechanism for the LD, which thus reduces the cost and power consumption of such an apparatus. Hence, it has been conventionally difficult to attain both the temperature stabilization of a fiber laser device and generation of light having a linear polarization, 6 W or more, and 1075 nm or less using a fiber laser device.                Patent Document 1: Japanese Patent No. 3012034        Non-patent Document 1: Japanese Journal of Applied Physics Vol. 43, No. 8B, 2004, pp. 5904-5906        Non-patent Document 2: Rare-earth-doped Fiber lasers and amplifiers, (Marcel Dekker, Inc., 2001), p. 145, FIG. 10        